1. Field of the Invention
This invention is related to the field of thermocouples and in particular to a three-wire self-diagnostic thermocouple which permits detection of calibration changes.
2. Description of the Related Art
A change in the calibration of a thermocouple can cause an error in the detection of the temperature to be determined or controlled. In particular, where equipment is being controlled by a thermocouple, this error could result in the equipment being damaged. Additionally, when the thermocouple controls a process, this error could result in the failure of the process to produce the desired product.
Currently, the accuracy of an installed thermocouple can be assured by placing a second thermocouple of known calibration beside the installed thermocouple and comparing the measured temperatures. This method is not always easy or possible to do and requires periodic checking to be effective. Additionally, it is not conveniently possible to know with certainty which of the two thermocouples is faulty when the two thermocouples indicate different temperatures.
The use of two thermocouple elements within a metal sheath is taught by Burley in U.S. Pat. No. 4,909,855, by Thom et al. in U.S. Pat. No. 4,778,537 and by Hollander in U.S. Pat. No. 5,111,002. Synder Jr. et al. in U.S. Pat. 4,224,461 teach a two wire thermocouple which includes a third wire as a ground wire. Petry in U.S. Pat. No. 2,696,118 teaches a temperature indicating device having two thermocouples in which the two thermocouples have a common element. Further, Bock in Canadian Patent 675,473 teaches that at least four thermocouple junctions are required to eliminate ambiguity from the measured temperature.